This invention relates to measuring intraocular pressure.
Nearly one percent of the total population of the United States suffers from a form of blindness known as Glaucoma. Glaucoma is characterized by an increase in pressure within the eye, which causes visual defects and ultimately may cause irreversible blindness. As the intraocular pressure rises to abnormal levels, damage is caused to the ocular nerve and surrounding retinal tissues. The patient seldom experiences any symptoms that might indicate that the disease exists until major damage occurs. Typically, the patient's intraocular pressure is elevated, the retinal field is seriously diminished, ocular nerve damage occurs and there may be some degree of pain.
As part of many standard eye examinations, a test of intraocular pressure (tonometry) is performed to detect the early stages of glaucoma.
A measure of the pressure within the eye is conventionally obtained by indenting to a given depth or flattening to a given extent a portion of a measurement surface of the eye, usually the cornea, and then determining the amount of force required to produce the given flattening or indentation. The flattening or indentation is resisted by the resiliency of the measurement surface and by the internal pressure of the eyeball. The determined force is then converted to a measurement of intraocular pressure.
Commonly, the flattening or indentation is produced by contacting the tip of an instrument directly onto the measurement surface, and then pressing the tip, whose dimensions are known, against the surface toward the eye. For such measurements to be accurate, the tip of the instrument must be properly oriented with respect to the measurement surface, and the directon of pressing toward the eye must be substantially normal to the measurement surface.
Measurement of intraocular pressure using conventional apparatus generally requires that the operator subjectively judge the depth, approach angle, and position of the measuring instrument upon the eye. Operation of such devices depends upon operator skill and consistency. Operator error and the combination of these subjective variables can result in variability in measurements taken by a particular operator from time to time, as well as inconsistency in measurements taken by different operators.
Many tonometers in use employ optical systems that allow the clinician to monitor the amount of applanation or indentation of the eye to adjust the proper angle and depth of penetration. Such devices typically include a lens that rests directly upon the eye, through which the operator views the tear meniscus to judge the correctness of the applanation at the time of measurement. Such devices typically require equipment for holding the patient's head in a particular position for a time, and often employ slit lamp equipment to aid in alignment. Some time is required to set up such apparatus preparatory to each measurement.
It is generally accepted that, when properly and skillfully used, direct contact tonometers can give more valid indications of intraocular pressure, and can provide more reliable diagnosis of early stages of glaucoma, than can other types of tonometers.
A variety of disease pathogens can be found on the surface of the eye, and particularly in the fluid film that covers the eye. These include, for example, pathogens causing herpes and, possibly, acquired immune deficiency syndrome (AIDS). A disadvantage of conventional direct contact tonometers is that because they must touch the eye, they can transmit such diseases from eye to eye and patient to patient.
It has been suggested that direct contact tonometers be provided with disposable prophylactic covers, for preventing transmission of disease pathogens from one eye to another. Many known tonometers, including those which the operator aligns by employing a lens in contact with the measurement surface, cannot be modified to accommodate such covers. Those devices that have been so modified are, at least partly as a result, not sufficiently sensitive to provide accurate measurements, and they have not been accepted by the medical community as clinically practical measurement devices.
Damage to the eye sometimes occurs, owing to individual tissue susceptibility to injury, to mishap, or to operator error.
Apart from error due to subjective judgments and error of the operator, measurement error often is an effect of the design of the particular device, and especially of the particular transduction scheme.
In early tonometers, lever systems were actuated by metallic springs or cams to provide a mechanical analog of the intraocular pressure. The precision of such devices depends upon the characteristics of compressibility of the spring systems and individual units produce differing measurements to the extent that their spring systems differ. Spring fatigue and changes in temperature can cause changes in measurement.
In other known other devices strain gauges are directly coupled to the measurement surface through a metal shaft that directly contacts the eye. These, too are affected by variations in temperature, and they can be plagued with signal conditioning problems and poor schemes for calibrating the strain gauge Bridges. Some such systems cannot be calibrated by the user and consistency in manufacture or materials cannot be assured in the commercial production of such instruments.
In other known tonometers a piezoelectric crystal transducer is directly coupled to a metallic plunger which directly contacts the eye. Piezoelectric crystals can be affected by small and practically undetectable changes in temperature. Because piezoelectric transducers respond not only to force but also to temperature, varying temperatures in ambient air as well as body heat transferred throuqh the shaft from the patient to the piezoelectric crystal can interfere with precise measurement of the intraocular pressure. Moreover, the piezoelectric crystal transducer can be affected not only by temperature but also by the velocity with which the force applied to the measurement surface changes. Such devices can yield a voltage analog signal that combines contributions of the applied force, the velocity at which force is increased, and the temperature of the test environment. Further, such devices typically require use of a microprocessor for calibration and for adjustment for the non-linearity of the transducer mechanism. The user cannot easily recalibrate the device in the field.
In devices known as "non-contact" or "airpuff" tonometers, compressed gases are directed at the cornea to flatten or indent it. These are referred to as "non-contact" devices because apart from one or more bursts of air fired or released toward the eye from a predetermined distance they do not come into contact with the measurement surface. Typically in such devices an incident light wave is transmitted by light emitting diodes to the cornea, which reflects it back to phototransducers within the device. As the measured pressure of the compressed gas jet is directed toward the measurement area, the surface is flattened and a measure of reflected light yields a relative measurement of intraocular pressure. Such devices are generally considered cumbersome to operate, and obtaining consistent measurements depends upon the skill and technique of the operator in aligning the instrument at the proper distance and orientation to the eye. They are generally regarded by health care workers as useful initial screening devices, but they are not generally accepted as providing accurate measurements. The major attribute of their acceptance has been the isolation of the patient from the device to inhibit disease transmission The patient often complains of the pain associated with the blast of air that must be delivered to the eye to obtain a measurement, and many patients are often reluctant to be measured a second time by such devices.